Just though I would share something that worked well for me. The brake/directional lenses on my K0 are the original Stanley's. The front lens look like they kissed some pavement some time in the past. How they didn't crack is beyond me, but they were left with some nasty scratches and abrasions bordering on gashes. I've seen replacements on eBay, but they were after-market. I was tempted to get some but thought I would try to restore the ones I had first.
Started with 400 wet/dry, then 1000 wet/dry and finished up with rubbing compound. They turned out far better than I imagined they would. It would take very close inspection to see any difference.